In general, a technology of a digital rights management (DRM) is used to protect a copy right of digital content. In an environment of the DRM described above, the content is encrypted and a key used to encrypt the content is managed by a license, so that only a legal user can decode and use the content. However, the encryption technology based a DRM technology has a limitation in that it cannot counteract an illegal distribution of decoded content or the like.
To compensate this, a study to track illegal distributors of the content by using a watermarking technique is being actively progressed.
The watermarking technique and forensic marking technique that are representative technologies to protect a copy right of multimedia content may be regarded as the same technology in terms of the method to insert or extract signals that cannot be recognized by a user into or from the content. In practice, the forensic mark is inserted and extracted by using insertion and extraction methods that are used in the existing watermarking method so that a purchaser cannot perceive it. However, there is a difference in that information inserted is different from one another between the watermarking technique and the forensic marking technique. It is because the aim is to track information on a legal purchaser from illegally distributed content by inserting copy right information of content in the watermarking technique and inserting purchaser information to purchase the content in the forensic marking technique.
Meanwhile, since different purchaser information is inserted in the forensic marking technique, the same content has different information inserted thereinto. A malicious purchaser may try to remove the inserted forensic mark using the difference from each content, which is referred to a collusion attack. Thus, an anti-collusion code that can counteract the collusion attack is being actively studied when generating the forensic mark.
In the conventional forensic marking technique that operates as described above, since it is utilized to track an illegal distributor for later use, there is a problem that it is not possible to prevent the illegal content from being distributed in advance.
Further, since the existing anti-collusion code is used without systemizing the total number of users and the number of detectable ones in collusion to a specific unit, the anti-collusion code becomes long in length in proportion to the total number of users and the number of detectable ones in collusion.
Also, although the forensic mark inserting and detecting technique uses a masking method to which a human visual system (HVS) or the like is applied in order to guarantee strength against various content transformations, it has a problem that a forensic mark inserting and detecting speed is decreased.